I. Field of the Invention
The disclosure relates to conversion of insoluble cellulosic biomass to biofuel or other useful products. More particularly, the disclosure pertains to the use of microorganisms and mechanical disruption to enhance cellulosic biomass solubilization and conversion.
II. Description of the Related Art
Cellulosic biomass is a relatively inexpensive, renewable and abundant material that can be used to generate fuels, chemicals, fibers, and energy. However, large-scale utilization of plant biomass is hindered, at least in part, by the lack of low-cost technologies capable of efficiently converting the biomass into soluble, reactive intermediates—notably including sugars. For example, most plant cell walls are resistant to digestion by cellulase enzymes.
Pretreatment of biomass may render the biomass more amenable to enzymatic digestion. Pretreatment may remove biomass components such as lignin and/or hemicellulose that impede access to cellulase enzymes. Pretreatment may also cause structural changes (e.g. particle size, porosity, surface area) in the biomass. Various biomass pretreatment technologies have been developed. Examples of these developments include use of dilute acids or bases, steam explosion, autohydrolyisis, controlled pH, AFEX, and aqueous ammonia pretreatment.
It is commonly believed that pretreatment of cellulosic biomass is required for biological conversion of cellulosic biomass. According to this common belief, pretreatment using high temperature and/or chemicals is necessary prior to biological processing in order to achieve the high solubilization yields upon subsequent biological processing that are generally required for economic viability.
However, capital and operating costs for pretreatment are substantial, and pretreatment further increases processing costs by negatively impacting the performance of downstream processing operations. In particular, all known pretreatments either produce compounds which inhibit hydrolysis and fermentation or require recovery of chemical reagents (for example, ammonia or ionic liquids), or both.
It is also believed that milling requires too much energy to be practical as a pretreatment prior to solubilization using fungal cellulase. It is disclosed here that (a) certain anaerobic cellulolytic microbes such as Clostridium thermocellum are capable of achieving significant solubilization of lignocellulose without pretreatment, (b) the resistance of such microbes to mechanical disruption, and (c) it is possible and beneficial to enhance solubilization by mechanical disruption of partially-solubilized solids—and in particular during microbial fermentation—termed co-treatment.